A question which has recieved comparatively less attention is how
surface roughness impact the self-diffusivity of moderate-to-dense
confined fluids. The goal of this paper is to answer this question
systematically via molecular simulations. The most obvious way in which roughness
influences dynamic properties is through the modification of fluid particle–wall
collisions. Here, we investigate how
different representations of this collisional modification lead to
changes in the self-diffusion of a simple, confined fluid. As a
starting point, we study the monodisperse hard-sphere (HS) fluid
confined between smooth hard walls. Despite its simplicity, we choose
the HS fluid because it captures many of the important effects
arising from the dominant excluded volume interactions in liquids [1]}, [2]} Obviously, a
smooth hard wall does not possess any roughness whatsoever, but it
still serves as a useful reference point for studying confined fluids,
specifically the dynamic properties in this paper. In fact, the slit
pore geometry is often assumed when determining the pore size
distribution from adsorption isotherms in real porous materials.[3]}, [4]}, [5]}, [6]}, [7]}
In our simulations, we introduce surface roughness in two
separate ways (i) by modifying the boundary conditions for collisions between fluid
particles and a flat wall surface and (ii) by giving the surface of the
confining walls physical roughness or shape. The former crudely
represents surface roughness on a length scale much smaller than the
diameters of the fluid particles (e.g., confined colloids), while the
latter models physical roughness on a length scale comparable to the
diameter of the fluid particles (e.g., confined molecular fluids).
While neither of the above types of walls can be said to
completely represent the roughness present in real physical systems,
they represent the types of roughness that can be incorporated in molecular
simulations, and serve as a good starting point to address the impact of
surface roughness on self-diffusivity.
